1. Field of the Invention
The present invention relates to a forging apparatus for an inner race of a constant velocity universal joint, which makes it possible to simplify the structure and decrease the number of parts.
2. Description of the Related Art
Conventionally, the inner race of the constant velocity universal joint is formed to have a ring-shaped configuration with its spherical outer circumference. A plurality of ball grooves are formed on the outer circumference. When the inner race of the constant velocity universal joint is produced by means of a forging process, the following procedure is adopted. That is, a material is arranged at the inside of a combined die which is split into two, i.e., an upper die and a lower die, because the inner race of the constant velocity universal joint has an undercut shape over its outer circumference. After that, the upper die and the lower die are clamped to carry out the forging process. However, this method is disadvantageous in that the upper die and the lower die tend to suffer centering deviation, resulting in decrease in accuracy of formation. In addition, a burr appears on a formed product between the upper die and the lower die. Therefore, it is necessary to remove the burr by means of cutting processing. For this reason, a problem arises in that the number of production steps is increased, and the production process requires a long period of time.
As a method for solving such a problem, Japanese Laid-Open Patent Publication No. 63-199040 discloses a press-forming method for an inner race of a constant velocity universal joint (hereinafter referred to as "conventional technique 1"). In this method, a material is placed on a lower punch, and then an upper die is lowered. Accordingly, a plurality of segmented dies are pressed by the upper die, and the segmented dies are displaced along tapered surfaces of a guide ring in a direction to make approach to a central axis of the material. Thus, the respective segmented dies make tight contact with each other. Subsequently, when an upper punch is lowered, the material is formed in conformity with a cavity which is constructed by the lower punch, the upper punch, and the segmented dies. Thus, the inner race of the constant velocity universal joint is obtained.
Japanese Laid-Open Patent Publication No. 57-56132 discloses an apparatus for forming an inner race of a constant velocity universal joint (hereinafter referred to as "conventional technique 2"), comprising a plurality of split dies which are held at the inside of a reinforcing ring by the aid of a tapered base and a die guide. A positioning means presses the plurality of split dies in accordance with the lowering action of a press slide. Accordingly, the split dies approach a central axis of a material in accordance with the guiding action of the tapered base, and a punch, which is disposed over the material, enters the material. Thus, the forging process is carried out.
However, the conventional technique 1 includes no mechanism for positioning the segmented dies in the circumferential direction. For this reason, when the respective segmented dies are allowed to make tight contact with each other, an error may arise concerning the position of the segmented die. There is a fear of decrease in accuracy of formation of the formed inner race of the constant velocity universal joint.
The conventional technique 2 requires the tapered base and the die guide. For this reason, the structure of the apparatus is complicated, and it is also necessary to highly accurately perform the production process and the assembling operation for respective parts. Therefore, a problem arises in that the production cost of the apparatus is expensive. Further, it is also necessary to perform the complicated operation for assembling the split dies to the reinforcing ring together with the tapered base and the die guide.
On the other hand, it is necessary to apply, in the downstream step, the mechanical processing such as the chamfering processing to the inner race of the constant velocity universal joint formed by the forging process. This procedure is shown in FIGS. 9 to 11. At first, an inner race 2 of a constant velocity universal joint is held by engaging a plurality of dampers 4 on the side of a small end section 2a of a wall for forming ball grooves 3 of the inner race 2 of the constant velocity universal joint (see FIG. 9). Subsequently, a hole 5 is defined at a central portion of the inner race 2 of the constant velocity universal joint by using a drill or the like. A tool 6a installed to an NC processing machine is used to apply finishing processing to a large end section 2b and an inner circumferential surface of the hole 5. A tool 6b is used to define a groove 7 on an inner circumferential surface of the hole 5. Thereafter, a collet damper 8 is inserted into the hole 5 from the side of the large end section 2b to hold the inner race 2 of the constant velocity universal joint (see FIG. 10). The tool 6a of the NC processing machine is used to apply finishing processing, planing processing, and chamfering processing to the small end section 2a. Further, finishing processing is applied to the ball grooves 3. As shown in FIG. 11, a chamfering processing machine equipped with a grinding wheel 9 is used to apply chamfering processing to both sides of each of the plurality of ball grooves 3 to form chamfered sections 3d. As described above, it is necessary to apply the chamfering processing to the respective ball grooves 3, resulting in increase in the number of regions to be processed. Therefore, it takes a long time to carry out the production process. Further, it is necessary to exchange the worn grinding wheel 9, resulting in a problem that the production cost is expensive. Furthermore, the small end section 3a, the back surface section 3b, and the large end section 3c of the ball groove 3 differ in configuration. Therefore, different grinding wheels 9 are required for these sections, resulting in a problem that the production cost is more expensive.